The following invention relates to an alternative and beneficial design of stator and/or rotor for a direct drive rotary generator, particularly a direct drive rotary generator which can be driven directly without the need for a gearbox and specifically for a generator in the form of an axial flux generator as disclosed in WO 2015/075456.
Such a direct drive generator is formed of an elongate cylindrical series of stator annuli and a coaxial elongate cylindrical series of rotor annuli and mounted for relative rotational movement around their common axis, the series of stator annuli interleaving the series of rotor annuli, and either of the rotor/stator annuli having a contiguous or substantially contiguous sequence of coils around its circumference, and the other of the rotor/stator annuli having a corresponding sequence of permanent magnets of alternating polarity spaced around its circumference and at the same pitch as that of the coils, the arrangement being such that lines of magnetic flux passing across the air gap between one magnet carrying annulus to the next cut the turns of the coils of the corresponding interleaved coil carrying annulus, and thus induce in the coils electromagnetic forces as the rotor is caused to rotate relative to the stator.
US 2008/231132 also discloses an axial flux generator and suggests ratios of magnets and coils which are suitable for directly generating three phase current which is often used for electricity transmission and distribution.
A key feature of the generator of WO 2015/075456 is that the pitch of the stator coils is selected to be the same as that of the rotor magnets. This ensures that the electromagnetic forces induced in each of the stator coils by the magnet carrying rotor are all in phase with one another. By this means, all of the outputs of the stator coils may conveniently be electrically connected to one another, so combining their respective outputs for feeding an electrical load.
However, a disadvantage arises from this arrangement. This is the effect known as cogging. Cogging is the variation in mechanical counter-torque which resists the applied torque used to turn the generator. In one form cogging arises as a result of the stator coils generating alternating current. Another severer form of cogging results from magnetic interaction between magnets of an armature and ferromagnetic material of the stator, for example ferrite material in the centre of coils of the stator.
US 2010/0194251 discloses an axial generator which works on different principles and in which paired spaced rotor disks each carry magnets so that pairs of magnets repel each other over a gap between the spaced rotor disks. A stator assembly in the gap comprises a series of coils with their centres filled with ferrite material. Such a system suffers from cogging originating from magnetic interaction between the magnets and the ferrite material. By using sixteen magnet pairs (and twelve coils) cogging due to magnetic interaction is reduced. However, no consideration is given to reduce any cogging resulting from the stator coils generating alternating current.
For the impellor of any wind turbine it is desirable to ensure that any torque ripple (resulting from the said cogging of either origin) arising from rotation of its generator is kept to a minimum practical. Any ripple “felt” by the blades of the impellor, even if the rotary inertia of the generator masks its inherent cogging in terms of fluctuations in its instantaneous rate of rotation, can be detrimental to their longevity, and even fatal if resonances occur. Ideally, cogging should be kept to 1% or less.
In addition, it is desirable for any generator used in such a wind turbine, especially one as is disclosed in WO 2015/075456 in which the magnets to be used are of the type known as ferrite, having weaker fields than the more commonly used type known as rare earth, that as much electrical energy as is possible be generated from the given volume occupied by the generator within the nacelle of the turbine.
To extract the maximum mechanical power from a wind turbine, it has been found beneficial to allow the blades of wind turbines to rotate at variable speed depending on the prevailing wind conditions. This flexibility is not possible in some conventional designs where for example, a three phase generator is driven by the wind turbine rotor and its output phases supply—and are locked onto—a national grid. In this case, the rate of rotation is dictated by the fundamental frequency of the grid (in the UK, 50 Hz, and USA 60 Hz).
Therefore there has been a move to process the electrical power generated by the generator by first rectifying it to direct current and then re converting the electrical power back e.g. to three phase suitable for feeding a national supply grid. In this case, the generator may then be permitted to run at a speed best suited for obtaining the maximum energy possible from the turbine blades.
In this case, the greater and smoother the power provided by the generator, the better.
Existing prior art (for example US 2008/231132) discloses the generation of three phase power. In this case, the useful energy, being the Root Mean Square, is 0.866 of the line peak. (In the case of a single phase system, it is 0.5 of the peak.)